Add reset / enhance set commands

[arina-ielchiieva]

1. Add reset command
   !reset all - resets all sqlline properties (deliberately did not allow to reset all with !reset (without arguments) since this is high prone to user to hit enter prematurely).
   !reset <property> - resets property to default.

2. Enhance set command
   Currently set command does the following:
   !set - lists all properties and their values
   !set <key> <value> - sets property to the given value
   Add:
   !set all - equivalent to !set to match !reset all syntax
   !set <property> - lists current property value

[arina-ielchiieva]

Opened PR - #144.

[julianhyde]

Fixed in 0b00709; thanks @arina-ielchiieva!